A polyvinyl acetal resin, its preparation method, its application, and a film containing the polyvinyl acetal resin.

Abstract

This invention discloses a polyvinyl alcohol acetal resin, its preparation method, its application, and a film containing the polyvinyl alcohol acetal resin. The absolute value of the difference between the chloride ion content and the acid value in the resin is ≤0.02. A 0.76 mm thick film made from this resin exhibits transmittance of not less than 85%, 88%, and 90% at wavelengths of 440 nm, 540 nm, and 640 nm, respectively, and an average transmittance of not less than 89.5% across the entire wavelength band under a D65 light source. The preparation method includes: dissolving polyvinyl alcohol and cooling it; adding an acid catalyst and n-butyraldehyde; carrying out an acetalization reaction using staged gradient stirring; neutralizing; and then performing multi-stage washing, dehydration, purification, and drying to obtain the resin. By monitoring the conductivity of the washing water online to ≤40 μS / cm, precise control of ionic impurities is achieved, thereby solving the color difference problem caused by the uneven transmittance of PVB resin at different wavelengths. This method is suitable for applications such as high-transparency laminated glass.

Description

Technical Field

[0001] This invention relates to the field of polymer materials technology, and particularly to a polyvinyl acetal resin, its preparation method, its application, and a film containing the polyvinyl acetal resin. Background Technology

[0002] Polyvinyl acetal (PVB) resin is a core material for laminated glass, photovoltaic modules, and high-end optical displays. Its optical performance directly affects the visual clarity and color accuracy of the final product. Currently, the industry generally uses the average transmittance of a wide-band visible light spectrum as the main indicator to evaluate the optical performance of PVB resin. While this method reflects the overall light transmittance level, it cannot accurately characterize the resin's transmittance characteristics in key regions of the visible spectrum. In particular, it struggles to identify and prevent common color difference defects such as varying degrees of haze and color change that are visible to the naked eye after the product is laminated with glass.

[0003] PVB resin is primarily white in its macroscopic state, but when it is made into a film and laminated with glass, microscopic color difference defects are significantly amplified, manifesting as uneven transmittance at different wavelengths. This results in the final product exhibiting color variations such as blue and yellow hues. Existing technologies and commercially available products typically only guarantee overall transmittance, neglecting transmittance uniformity at specific wavelengths. Consequently, although the overall transmittance of the film meets the standards, its optical quality is uneven, affecting high-end applications.

[0004] Therefore, there is an urgent need for a polyvinyl alcohol acetal resin that has a uniform transmittance at different wavelengths and does not exhibit color differences such as blue or yellow hues. Summary of the Invention

[0005] The purpose of this invention is to provide a polyvinyl acetal resin to solve the problem in the prior art where the transmittance of polyvinyl acetal resin is uneven at different wavelengths, resulting in color differences such as blue and yellow hues.

[0006] The present invention also aims to provide a preparation method for preparing polyvinyl acetal resin with uniform transmittance and excellent optical quality at different wavelengths.

[0007] Another objective of this invention is to provide an application of polyvinyl acetal resin in safety laminated glass.

[0008] Another objective of this invention is to provide a film prepared from polyvinyl acetal resin.

[0009] In a first aspect, the present invention provides a method for preparing polyvinyl acetal resin, characterized by comprising the following steps: S1. Add polyvinyl alcohol resin to pure water, heat to 90-100℃, and stir to obtain a polyvinyl alcohol solution; S2. Cool the polyvinyl alcohol solution to 10-30℃, add acid catalyst and n-butyraldehyde to carry out acetalization reaction. After the reaction is completed, add alkali solution to the reaction system to neutralize to pH 3-5 to obtain resin slurry. S3. After washing, dehydrating, purifying and drying the neutralized resin slurry, polyvinyl alcohol acetal resin is obtained.

[0010] By adopting the above technical solutions, heating to 90-100℃ in S1 allows the polyvinyl alcohol resin to fully dissolve in pure water, forming a homogeneous and stable polyvinyl alcohol solution. This lays the foundation for the uniformity of the subsequent acetalization reaction and avoids uneven resin structure due to insufficient dissolution, which would affect light transmittance. In S2, cooling to 10-30℃ before the acetalization reaction controls the initial reaction rate and prevents excessively vigorous local reactions that could cause structural defects. The acid catalyst efficiently catalyzes the acetalization reaction between polyvinyl alcohol and n-butyraldehyde. Neutralizing to a pH of 3-5 terminates the reaction, while avoiding an overly alkaline environment that could damage the resin. Lipid degradation; S3 effectively removes residual acid radicals, chloride ions and other impurities through staged water washing, dehydration and precise purification control. In S2, the acetalization reaction is carried out through staged gradient plasticizing and stirring to adapt to the viscosity changes of the system at different stages of the reaction, ensuring uniform and thorough reaction. S3 uses a water washing process that combines primary washing with hot water and fine washing with high-purity water to remove different types of impurities. The conductivity is monitored in real time during dehydration. Purification is based on the standard that the conductivity of the washing water is ≤40μS / cm, which precisely controls the amount of residual impurities and ultimately achieves the balance of ion content in the resin, solving the problem of uneven light transmittance.

[0011] Preferably, the acetalization reaction includes the following steps: After adding n-butyraldehyde, stir at 1300-1500 rpm for 10-30 minutes to ensure uniform mixing; then increase the stirring speed to 1500-1850 rpm and stir at 10-20℃ for 1-2 hours; finally, raise the temperature to 50-60℃ at a heating rate of 10-20℃ / h, while increasing the stirring speed to 1800-1950 rpm, and keep warm for 2-4 hours.

[0012] Preferably, the water washing includes the following steps: The neutralized resin slurry is transferred to a water washing device. First, it is washed 3 to 5 times with water at 40 to 70°C to remove water-soluble impurities. Then, it is washed 3 to 5 times with high-purity water at 20 to 25°C with a conductivity ≤10μS / cm.

[0013] Preferably, dehydration includes the following steps: After each wash, the slurry is separated into solid and liquid components, and the washing waste liquid is discharged. The conductivity of the washing water discharged from the last stage of separation is monitored in real time using an online conductivity meter.

[0014] Preferably, the conductivity of the purified and dehydrated wash water is ≤40μS / cm.

[0015] Preferably, the polyvinyl alcohol resin has an average degree of polymerization of 1500 to 2500 and a degree of alcoholysis ≥ 99.0 mol.

[0016] More preferably, the average degree of polymerization of polyvinyl alcohol is 1700 to 1800.

[0017] Preferably, the amount of n-butyraldehyde added is 50% to 60% of the mass of polyvinyl alcohol resin.

[0018] Preferably, the acid catalyst includes hydrochloric acid and sulfuric acid.

[0019] Secondly, the present invention also provides a polyvinyl acetal resin, wherein the absolute value of the difference between the chloride ion content and the molecular weight concentration of the acid contained in the resin is ≤0.02.

[0020] By adopting the above technical solutions, the absolute value of the difference between the chloride ion content and the acid molecular weight concentration in polyvinyl acetal resin is ≤0.02. This indicator precisely controls the relative content of key impurity ions in the resin. Chloride ions and acid radical ions are the core factors causing uneven resin transmittance. An imbalance in their content will cause differences in the resin's absorption of light of different wavelengths, resulting in color differences such as blue and yellow hues in the film. This indicator, by strictly limiting the balance of ion content, ensures that the resin absorbs light evenly across the entire visible light spectrum, providing a core guarantee for the film's transmittance in the blue, green, and red light regions to meet the set requirements, thus fundamentally solving the color difference problem of existing technologies.

[0021] Thirdly, the present invention also provides a polyvinyl acetal resin film, comprising polyvinyl acetal resin and a plasticizer; The amount of plasticizer used is 25-30 wt% of polyvinyl alcohol resin.

[0022] Preferably, the plasticizers include triethylene glycol diisooctanoate (3G8) and tricresyl phosphate.

[0023] Preferably, the polyvinyl acetal resin film is prepared by hot pressing at a temperature of 150–160°C.

[0024] Preferably, when the thickness of the polyvinyl acetal resin film is 0.76 mm, the transmittance is ≥85% at wavelengths of 450–435 nm, ≥88% at wavelengths of 577–492 nm, ≥90% at wavelengths of 760–622 nm, and the average transmittance across the entire wavelength range is ≥89.5% under a D65 light source.

[0025] More preferably, when the thickness of the polyvinyl acetal resin film is 0.76 mm, the transmittance at a wavelength of 440 nm is ≥85%, the transmittance at a wavelength of 540 nm is ≥88%, the transmittance at a wavelength of 640 nm is ≥90%, and the average transmittance across the entire wavelength band is ≥89.5% under a D65 light source.

[0026] Fourthly, the present invention also provides an application in safety laminated glass.

[0027] The beneficial effects of this invention are: 1. This invention establishes a color difference evaluation system for PVB resin. PVB resin mainly appears white in its macroscopic state, but color differences are easily amplified after film formation. Regarding the efficient characterization of color difference in PVB resin products, this patent proposes an independent evaluation system for three characteristic wavelengths: blue, green, and red. Based on a large amount of data, it clarifies the minimum transmittance requirements for different wavelengths, thereby enabling more scientific, accurate, and efficient detection of color differences in PVB resin.

[0028] 2. This patent, starting from the principle affecting the color difference of PVB resin, through systematic research, first clarified the key factors affecting the color difference of PVB resin. For example, the content of chloride ion impurities is the key to controlling the fogging and color change of PVB resin. Subsequently, starting from the resin synthesis and post-processing, through staged gradient speed stirring and water washing process depth control, the precise control of ion content was achieved, keeping the content difference within an extremely narrow range. This effectively ensured high transmittance in the blue light region, solved the color change problem of PVB resin, and further improved the high transmittance of PVB resin in the green and red light regions. Thus, it prepared a product with high transmittance across the entire visible light spectrum, achieving spectral uniformity.

[0029] 3. This patent, starting from the engineering production of PVB resin, proposes quantitatively controllable production process conditions. Specifically, by using a staged, gradient-increase stirring reaction and online monitoring of the conductivity of the washing water, controlling it to ≤40 μS / cm, precise control of ion content can be achieved, solving the problem of untimely ion detection and ultimately realizing the stable production of high-transparency PVB resin. The technology provided by this patent has high practicality and competitiveness, and can significantly improve production efficiency and enhance the product quality of PVB resin. Attached Figure Description

[0030] Figure 1 This is an image of the high-transparency PVB film prepared in Example 1 of the present invention. Figure 2 This is a photograph of the appearance of the bluish PVB film prepared in Comparative Example 2 of the present invention. Detailed Implementation

[0031] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, specific embodiments of the present invention will be described in detail below.

[0032] A method for preparing polyvinyl acetal resin, characterized by comprising the following steps: S1. Add polyvinyl alcohol resin to pure water, heat to 90-100℃, and stir to obtain a polyvinyl alcohol solution; S2. Cool the polyvinyl alcohol solution to 10-30℃, add acid catalyst and n-butyraldehyde to carry out acetalization reaction. After the reaction is completed, add alkali solution to the reaction system to neutralize to pH 3-5 to obtain resin slurry. S3. After washing, dehydrating, purifying and drying the neutralized resin slurry, polyvinyl alcohol acetal resin is obtained.

[0033] By adopting the above technical solutions, heating to 90-100℃ in S1 allows the polyvinyl alcohol resin to fully dissolve in pure water, forming a homogeneous and stable polyvinyl alcohol solution. This lays the foundation for the uniformity of the subsequent acetalization reaction and avoids uneven resin structure due to insufficient dissolution, which would affect light transmittance. In S2, cooling to 10-30℃ before the acetalization reaction controls the initial reaction rate and prevents excessively vigorous local reactions that could cause structural defects. The acid catalyst efficiently catalyzes the acetalization reaction between polyvinyl alcohol and n-butyraldehyde. Neutralizing to a pH of 3-5 terminates the reaction, while avoiding an overly alkaline environment that could damage the resin. Lipid degradation; S3 effectively removes residual acid radicals, chloride ions and other impurities through staged water washing, dehydration and precise purification control. In S2, the acetalization reaction is carried out through staged gradient plasticizing and stirring to adapt to the viscosity changes of the system at different stages of the reaction, ensuring uniform and thorough reaction. S3 uses a water washing process that combines primary washing with hot water and fine washing with high-purity water to remove different types of impurities. The conductivity is monitored in real time during dehydration. Purification is based on the standard that the conductivity of the washing water is ≤40μS / cm, which precisely controls the amount of residual impurities and ultimately achieves the balance of ion content in the resin, solving the problem of uneven light transmittance.

[0034] In some embodiments, the acetalization reaction includes the following steps: After adding n-butyraldehyde, stir at 1300-1500 rpm for 10-30 minutes to ensure uniform mixing; then increase the stirring speed to 1500-1850 rpm and stir at 10-20℃ for 1-2 hours; finally, raise the temperature to 50-60℃ at a heating rate of 10-20℃ / h, while increasing the stirring speed to 1800-1950 rpm, and keep warm for 2-4 hours.

[0035] By adopting the above technical solution and employing a staged gradient stirring speed, the system characteristics of each stage of the acetalization reaction are adapted. In the initial stage, the low stirring speed of 1300-1500 rpm ensures thorough mixing of n-butyraldehyde and polyvinyl alcohol solution, avoiding uneven resin particle formation caused by excessively high local concentrations. In the middle stage, the stirring speed is increased to 1500-1850 rpm after heating to address the thickening phenomenon caused by the reaction, enhance mass transfer, and ensure reaction uniformity. In the later stage, the temperature is further increased and the stirring speed is increased to 1800-1950 rpm to promote the depth of the acetalization reaction, break the mass transfer limitations of the high-viscosity system, ensure complete reaction, reduce unreacted monomers and residual hydroxyl groups, avoid transmittance fluctuations caused by uneven resin structure, and provide a guarantee for the uniformity of optical performance.

[0036] In some embodiments, washing includes the following steps: The neutralized resin slurry is transferred to a water washing device. First, it is washed 3 to 5 times with water at 40 to 70°C to remove water-soluble impurities. Then, it is washed 3 to 5 times with high-purity water at 20 to 25°C with a conductivity ≤10μS / cm.

[0037] By adopting the above technical solutions and employing a two-stage water washing process of primary washing and final fine washing, efficient removal of impurities is achieved. Hot water at 40–70°C can increase the solubility of water-soluble impurities (such as salts generated in the reaction and excess acid catalysts) and accelerate impurity desorption. 3–5 primary washes can quickly remove most of the easily soluble impurities, reducing the load on subsequent fine washing. High-purity water at 20–25°C (conductivity ≤10μS / cm) has high purity, avoiding the introduction of new impurities. Low temperature can reduce the swelling of resin particles and prevent structural damage. 3–5 fine washes can deeply remove residual trace amounts of chloride ions and other impurities that affect optical performance, providing process support for precise control of resin ion content.

[0038] In some embodiments, dehydration includes the following steps: After each wash, the slurry undergoes solid-liquid separation, and the washing waste liquid is discharged. The conductivity of the washing water discharged from the final stage of separation is monitored in real time using an online conductivity meter. Solid-liquid separation and waste liquid discharge after each wash prevent impurities from being re-mixed into the resin slurry, ensuring the washing effect. The real-time monitoring of the conductivity of the final stage washing water by the online conductivity meter can directly reflect the content of impurity ions in the washing water, providing real-time data support for determining the endpoint of the washing process, allowing for timely adjustment of the number of washes and parameters, avoiding insufficient or excessive washing, and ensuring the stability and consistency of resin purification effect.

[0039] In some embodiments, the conductivity of the purified and dehydrated wash water is ≤40 μS / cm. Purification is quantified by a conductivity of ≤40 μS / cm for the dehydrated wash water, as conductivity is directly related to the content of residual ionic impurities in the resin. When the conductivity of the wash water meets this requirement, it indicates that impurities such as chloride ions and acid radicals in the resin that affect color difference have been reduced to extremely low levels. This ensures that the absolute value of the difference between the chloride ion content and the acid molecular weight concentration in the resin is ≤0.02, thereby guaranteeing that the transmittance of the subsequent film meets the standards at each characteristic wavelength. This process locks in the optical quality of the resin and avoids uneven transmittance and color defects caused by residual impurities.

[0040] In some embodiments, the average degree of polymerization of the polyvinyl alcohol resin is 1500–2500, and the degree of hydrolysis is ≥99.0 mol%; further, the average degree of polymerization of the polyvinyl alcohol is 1700–1800; limiting the average degree of polymerization of the polyvinyl alcohol resin to 1500–2500 ensures that the viscosity of the acetalization reaction system is suitable, which facilitates stirring and mass transfer, and also imparts good film-forming properties and mechanical strength to the resin; a degree of hydrolysis ≥99.0 mol% means that the resin has sufficient hydroxyl content, which can fully react with n-butyraldehyde to increase the degree of acetalization, reduce light absorption defects caused by residual hydroxyl groups, and improve light transmittance.

[0041] In some embodiments, the amount of n-butyraldehyde added is 50% to 60% of the mass of polyvinyl alcohol resin. This ratio ensures the sufficiency of the acetalization reaction. Excess n-butyraldehyde allows the hydroxyl groups on the polyvinyl alcohol molecular chain to undergo acetalization to the maximum extent, reducing residual hydroxyl groups, decreasing the resin's absorption of light of a specific wavelength, and avoiding the decrease in light transmittance and uneven color caused by insufficient acetalization. Excess n-butyraldehyde can be completely removed by subsequent water washing processes without leaving any residue that affects product performance. This addition range balances reaction efficiency and product quality, ensuring that the resin's optical properties meet the standards.

[0042] In some embodiments, the acid catalyst includes hydrochloric acid and sulfuric acid.

[0043] A polyvinyl acetal resin, wherein the absolute value of the difference between the chloride ion content and the molecular weight concentration of the acid contained in the resin is ≤0.02.

[0044] By adopting the above technical solutions, the absolute value of the difference between the chloride ion content and the acid molecular weight concentration in polyvinyl acetal resin is ≤0.02. This indicator precisely controls the relative content of key impurity ions in the resin. Chloride ions and acid radical ions are the core factors causing uneven resin transmittance. An imbalance in their content will cause differences in the resin's absorption of light of different wavelengths, resulting in color differences such as blue and yellow hues in the film. This indicator, by strictly limiting the balance of ion content, ensures that the resin absorbs light evenly across the entire visible light spectrum, providing a core guarantee for the film's transmittance in the blue, green, and red light regions to meet the set requirements, thus fundamentally solving the color difference problem of existing technologies.

[0045] A polyvinyl acetal resin film includes polyvinyl acetal resin and a plasticizer; By adopting the above technical solutions, the film is made of polyvinyl acetal resin and plasticizer. The plasticizer can reduce the glass transition temperature of the resin and improve the processing fluidity of the resin, making the film easier to form during hot pressing and obtaining a flat and dense structure. The good compatibility between the plasticizer and the resin can avoid the increase in haze caused by phase separation, ensuring excellent light transmittance of the film, while improving the flexibility and mechanical properties of the film, meeting the requirements of laminated glass and other application scenarios for film processing and performance.

[0046] The amount of plasticizer used is 25-30 wt% of polyvinyl alcohol resin. This ratio can balance the optical and mechanical properties of the film. An appropriate amount of plasticizer can ensure that the film has good flexibility and processability, and avoid the film being brittle and easy to crack due to insufficient plasticizer. At the same time, it can avoid the film's light transmittance and haze due to excessive plasticizer, ensuring that the film maintains excellent optical transparency while having mechanical properties suitable for practical applications, thus achieving a unity of function and practicality.

[0047] In some embodiments, the plasticizer includes triethylene glycol diisooctanoate (3G8) and tricresyl phosphate.

[0048] In some embodiments, the polyvinyl acetal resin film is prepared by hot pressing at a temperature of 150–160°C. The film is prepared by hot pressing at 150–160°C, a temperature range that allows the resin and plasticizer to fully fuse, melting and molding the resin particles to form a uniform and dense film. This avoids the presence of pores and defects inside the film due to excessively low temperatures, which would affect light transmittance. Simultaneously, this temperature is below the resin's thermal decomposition temperature, preventing resin degradation or discoloration and ensuring the film maintains good optical performance and appearance quality. Precise control of the hot pressing temperature is a key process guarantee for obtaining a high-transparency, defect-free film.

[0049] In some embodiments, when the thickness of the polyvinyl acetal resin film is 0.76 mm, the transmittance is ≥85% at wavelengths of 450–435 nm, ≥88% at wavelengths of 577–492 nm, and ≥90% at wavelengths of 760–622 nm. Under a D65 light source, the average transmittance across the entire wavelength band is ≥89.5%. Furthermore, when the thickness of the polyvinyl acetal resin film is 0.76 mm, the transmittance at a wavelength of 440 nm is ≥85%, at a wavelength of 540 nm is ≥88%, at a wavelength of 640 nm is ≥90%, and under a D65 light source, the average transmittance across the entire wavelength band is ≥89.5%.

[0050] By adopting the above technical solutions, the transmittance indicators of the 0.76mm thick film in the key wavelength range and across the entire spectrum are clearly defined. 440nm (blue light region), 540nm (green light region), and 640nm (red light region) are the core characteristic wavelengths of visible light, covering the main spectral regions sensitive to human eyes. The minimum transmittance requirements for each wavelength ensure that the transmittance of the film is balanced under different wavelengths, avoiding blue and yellow color differences caused by low transmittance in a single wavelength band. An average transmittance of ≥89.5% across the entire spectrum under a D65 light source guarantees the overall transmittance level of the film, meeting the requirements of high-end applications such as laminated glass and photovoltaic modules for visual clarity and color fidelity. This fully realizes the core objective of the invention: solving the color difference problem of existing technologies and improving optical quality.

[0051] The specific embodiments of the present invention will be described in detail below, but it should be understood that the scope of protection of the present invention is not limited to the specific embodiments.

[0052] Example

[0053] Example 1: A polyvinyl acetal resin film was prepared by the following steps: S1. Add 1000 g of polyvinyl alcohol with an average degree of polymerization of 1700 and a degree of alcoholysis of 99.8% and 10 L of pure water to the reactor, heat to 95℃, and stir for 3 h to obtain a homogeneous PVA solution. S2. Cool the PVA solution to 20°C, add 31% hydrochloric acid and 55% n-butyraldehyde (based on the mass of PVA), and carry out the acetalization reaction according to the following gradient control procedure: Mix at 1300 rpm for 20 minutes at 10°C; At 15°C, the stirring speed was gradually increased to 1850 rpm, and the reaction was continued for 1.5 hours. Then, the temperature is increased to 55°C at a rate of 15°C / h, and the stirring rate is continuously increased stepwise to 1950 rpm during this period. The temperature is then maintained at 55°C for 3 hours to complete the deep reaction. After the reaction is complete, add 0.1M sodium hydroxide solution until the pH value is 4 to obtain resin slurry; S3. The neutralized resin slurry is then washed, dehydrated, purified, and dried. First, wash three times with hot water at 55℃. After each wash, centrifuge to dry and drain the washing waste liquid. Then wash three times with room temperature high-purity water with a conductivity of 8 μS / cm (25℃), and centrifuge and spin dry after each wash; The conductivity of the washing water discharged in the last stage during the washing process is monitored in real time by an online conductivity meter, and the conductivity at the end of the washing process is controlled to be stable at 38 μS / cm. The wet resin that has been washed to meet the standards is dehydrated, air-dried, and pulverized to obtain polyvinyl alcohol acetal resin powder. S4. Take the dried resin and mix it with 27% 3G8 plasticizer, and press it at 160°C to form a standard film with a thickness of 0.76 mm.

[0054] Example 2: A polyvinyl acetal resin film, which differs from Example 1 only in that it is washed four times with hot water at 55°C in S3, and the final drainage conductivity is controlled to be stable at 32 μS / cm.

[0055] Example 3: A polyvinyl acetal resin film, which differs from Example 1 only in that the average degree of polymerization of polyvinyl alcohol is 1800, and the final drainage conductivity is controlled and stabilized at 36 μS / cm.

[0056] Example 4: A polyvinyl acetal resin film, the only difference from Example 1 is that the washing with hot water at 55°C three times in S3 is replaced with washing with hot water at 65°C three times, and the final drainage conductivity is controlled and stabilized at 37 μS / cm.

[0057] Comparative Example

[0058] Comparative Example 1: A polyvinyl acetal resin film was prepared by the following steps: S1. Add 1000 g of polyvinyl alcohol with an average degree of polymerization of 1700 and a degree of alcoholysis of 99.8% and 10 L of pure water to the reactor, heat to 95℃, and stir for 3 h to obtain a homogeneous PVA solution. S2. Cool the PVA solution to 20℃, add 31% hydrochloric acid and 55% n-butyraldehyde (based on the mass of PVA), and carry out an acetalization reaction: The reaction was carried out at 55℃ and a constant rotation speed of 1500 rpm for 4.8 hours. After the reaction is complete, add 0.1M sodium hydroxide solution until the pH value is 4 to obtain resin slurry; S3. The neutralized slurry is subjected to a traditional water washing process without online conductivity monitoring and gradient plasticizing stirring. That is, after the reaction, it is neutralized to pH=4, and then the slurry is repeatedly washed with 25℃ water until the water is visible to the naked eye. The final drainage conductivity is about 68μS / cm.

[0059] Comparative Example 2, a polyvinyl acetal resin film, differs from Example 1 only in that the water washing in S3 is only done twice with warm water at 35°C, and the final drainage conductivity is 62 μS / cm.

[0060] Comparative Example 3: A polyvinyl acetal resin film was prepared by the following steps: S1. Add 1000 g of polyvinyl alcohol with an average degree of polymerization of 1700 and a degree of alcoholysis of 99.8% and 10 L of pure water to the reactor, heat to 95℃, and stir for 3 h to obtain a homogeneous PVA solution. S2. Cool the PVA solution to 20°C, add 31% hydrochloric acid and 55% n-butyraldehyde (based on the mass of PVA), and carry out the acetalization reaction according to the following gradient control procedure: The reaction was carried out at 55℃ and a constant rotation speed of 1500 rpm for 4.8 hours. After the reaction is complete, add 0.1M sodium hydroxide solution until the pH value is 4 to obtain resin slurry; S3. The neutralized resin slurry is then washed, dehydrated, purified, and dried. First, wash three times with hot water at 55℃. After each wash, centrifuge to dry and drain the washing waste liquid. Then wash three times with room temperature high-purity water with a conductivity of 8 μS / cm (25℃), and centrifuge and spin dry after each wash; The conductivity of the washing water discharged in the last stage during the washing process is monitored in real time by an online conductivity meter, and the conductivity at the end of the washing process is controlled to be stable at 40. The wet resin that has been washed to meet the standards is dehydrated, air-dried, and pulverized to obtain polyvinyl alcohol acetal resin powder. S4. Take the dried resin and mix it with 27% 3G8 plasticizer, and press it at 160°C to form a standard film with a thickness of 0.76 mm.

[0061] Comparative Example 4, a polyvinyl acetal resin, differs from Example 1 only in step S3, where centrifugation is not performed after each water wash, and separation relies solely on natural sedimentation; the conductivity at the end of the water wash is 45 μS / cm.

[0062] Performance testing: 1. Chloride ion content test: The chloride ion content in the resin was determined by silver nitrate titration. 2. Acid value test: The acid value of the resin is determined by acid-base titration, expressed as mg KOH / g; 3. Conductivity test: The conductivity was measured at a constant temperature of 25℃ using a calibrated conductivity meter. 4. Transmittance Test: Using a UV-Vis spectrophotometer (UV-8000A) at 25℃ with air as a reference, the transmittance of the above-mentioned films at wavelengths of 440 nm, 540 nm, and 640 nm was measured. The average of three measurements was taken, and the full-band spectrum of 380-780 nm was scanned. Based on the scan data, the average transmittance across the entire band under a D65 standard light source was calculated, and the lowest transmittance value across the entire band was recorded. Each sample was tested three times, and the average value was taken. 5. Appearance evaluation: Test conditions: In a dark room environment, a 0.76 mm standard film is illuminated from the back using a white light source, and its light transmission and color are observed from the front and compared with the standard sample.

[0063] Table 1 Performance test results

[0064] This patent establishes a mechanism for regulating ion content and spectral uniformity, and achieves this through a quantifiable process. The invention discovers that residual chloride ions and other impurities selectively absorb blue light (440nm), which is key to the blue hue difference in the thin film. To address this, the invention employs staged gradient-increase stirring to ensure a uniform reaction system and avoid localized impurity enrichment. More importantly, by using a final washing water conductivity ≤40 μS / cm as the online monitoring endpoint, precise and real-time control of the degree of ion impurity removal is achieved. Test data validates this mechanism; in the examples, |Cl... - Content-acid value|≤0.02 corresponds to a transmittance of ≥86% at 440nm, with balanced transmittance across all three wavelengths and no color difference; while the comparative example has excessive ion content, resulting in a significant deterioration in blue light transmittance and a noticeably bluish appearance. This indicates that limiting the ion difference to an extremely narrow range through process control is the fundamental reason for ensuring that PVB resin achieves balanced high transmittance across the entire wavelength range and eliminating color difference.

[0065] The above-disclosed embodiments are merely a few specific examples of the present invention. However, the embodiments of the present invention are not limited thereto, and any variations that can be conceived by those skilled in the art should fall within the protection scope of the present invention.

Claims

1. A method for preparing polyvinyl acetal resin, characterized in that, Includes the following steps: S1. Add polyvinyl alcohol resin to pure water, heat to 90-100℃, and stir to obtain a polyvinyl alcohol solution; S2. Cool the polyvinyl alcohol solution to 10-30℃, add acid catalyst and n-butyraldehyde to carry out acetalization reaction. After the reaction is completed, add alkali solution to the reaction system to neutralize to pH 3-5 to obtain resin slurry. S3. After washing, dehydrating, purifying and drying the neutralized resin slurry, polyvinyl alcohol acetal resin is obtained.

2. The preparation method according to claim 1, characterized in that, The acetalization reaction includes the following steps: After adding n-butyraldehyde, stir at 1300-1500 rpm for 10-30 minutes to ensure uniform mixing; then increase the stirring speed to 1500-1850 rpm and stir at 10-20℃ for 1-2 hours; finally, raise the temperature to 50-60℃ at a heating rate of 10-20℃ / h, while increasing the stirring speed to 1800-1950 rpm, and keep warm for 2-4 hours.

3. The preparation method according to claim 1, characterized in that, The water washing includes the following steps: The neutralized resin slurry is transferred to a water washing device. First, it is washed 3 to 5 times with water at 40 to 70°C to remove water-soluble impurities. Then, it is washed 3 to 5 times with high-purity water at 20 to 25°C with a conductivity ≤10μS / cm.

4. The preparation method according to claim 1, characterized in that, The dehydration process includes the following steps: After each wash, the slurry is separated into solid and liquid components, and the washing waste liquid is discharged. The conductivity of the washing water discharged from the last stage of separation is monitored in real time using an online conductivity meter.

5. The preparation method according to claim 1, characterized in that, The purified and dehydrated wash water has a conductivity of ≤40μS / cm.

6. The preparation method according to claim 1, characterized in that, The polyvinyl alcohol resin has an average degree of polymerization of 1500 to 2500 and a degree of alcoholysis ≥ 99.0 mol.

7. The preparation method according to claim 1, characterized in that, The amount of n-butyraldehyde added is 50% to 60% of the mass of polyvinyl alcohol resin.

8. The preparation method according to claim 1, characterized in that, The acid catalysts include hydrochloric acid and sulfuric acid.

9. A polyvinyl alcohol acetal resin, prepared by the preparation method according to any one of claims 1-8, characterized in that, The absolute value of the difference between the chloride ion content and the molecular weight concentration of the acid contained in the polyvinyl acetal resin is ≤0.

02.

10. A polyvinyl acetal resin film, characterized in that, Includes the polyvinyl acetal resin and plasticizer as described in claim 9; The amount of plasticizer used is 25-30 wt% of polyvinyl alcohol resin.

11. The polyvinyl acetal resin film according to claim 10, characterized in that, The film is prepared by hot pressing at a temperature of 150–160°C.

12. The polyvinyl acetal resin film according to claim 10, characterized in that, When the thickness of the polyvinyl acetal resin film is 0.76 mm, the transmittance is ≥85% at wavelengths of 450–435 nm, ≥88% at wavelengths of 577–492 nm, and ≥90% at wavelengths of 760–622 nm. Under a D65 light source, the average transmittance across the entire wavelength band is ≥89.5%.

13. A type of safety laminated glass, characterized in that, Includes the polyvinyl acetal resin according to claim 9.

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